EngineSerializerJsonDeserializeFromFileT - метод

public T JsonDeserializeFromFile<T>(
	string filePath
)
where T : class

Public Function JsonDeserializeFromFile(Of T As Class) ( 
	filePath As String
) As T

Dim instance As EngineSerializer
Dim filePath As String
Dim returnValue As T

returnValue = instance.JsonDeserializeFromFile(filePath)

JScript не поддерживает универсальные типы и методы.